Method of sulphurizing steel mold parts and parts produced thereby

ABSTRACT

An aluminium or aluminium casting is made by a casting machine using components of a ferreous material the portions of which come into contact with the molten aluminium or aluminium alloy during the casting have sulphurized surface layers which are highly resistant to the corrosive attack of the molten metal.

United States Patent [191 Niimi et al.

[4 June 18, 1974 1 METHOD OF SULPHURIZING STEEL MOLD PARTS AND PARTS PRODUCED THEREBY [56] References Cited UNITED STATES PATENTS 2,639,245 5/1953 Baxter 148/624 2,715,083 8/1955 Baxter 148/624 3,144,359 8/1964 Takahashi... 148/624 X 3,321,338 5/1967 Caubet.... 7. 148/624 X 3,623,919 11/1971 Suzuki 148/614 R 3,680,626 8/1972 Kusunoki et al 164/72 X Primary ExaminerRobert D. Baldwin Assistant Examiner,lohn E. Roethel Attorney, A en o Fi m 9! M w y ABSTRACT An aluminium or aluminium casting is made by a casting machine using components of a ferreous material the portions of which come into contact with the molten aluminium or aluminium alloy during the casting have sulphurized surface layers which are highly resistant to the corrosive attack of the molten metal.

11 Claims, 2 Drawing Figures SHEU i 0F 2 Illll METHOD OF SULPHURIZING STEEL MOLD PARTS AND PARTS PRODUCED THEREBY BACKGROUND OF THE INVENTION This invention relates to a method of casting aluminium alloy using a casting machine whose components which have surface layers which are highly corrosionresistant to molten aluminium or aluminium alloy, and a casting machine comprising such components.

Aluminium or aluminium alloy in the molten state has a strong tendency to react with various metals. In the casting of aluminium or its alloys, therefore, the surface portions of the casting equipment that are brought into contact with the molten metal are rapidly and seriously corroded. This necessitates premature replacement of the corroded parts, which results in a decreased operation efficiency and an increased casting cost. Also, the corroded surface layers of the components can fall off as impurities into the aluminium or aluminium alloy castings, thus adversely affecting the quality of the products.

In an attempt at extending the life of the casting equipment, it has hitherto been proposed and practiced to make the melt-contacting parts from cast iron having a black skin and coated with a non-metallic material,

such as alumina, talc, graphite, or zinc flower, which is highly resistant to molten aluminium, or to make those parts from ceramics. However, the former does not have adequate durability, with a life between about 100 and 200 hours, and the coating material tends to come off from the part surfaces in the casting operation. The latter also has a disadvantage in that the ceramics lack strength and are easily broken during operation.

SUMMARY OF THE INVENTION It is a primary object of the present invention to improve the efficiency and economy of the aluminium or aluminium alloy casting operation.

Another object of the invention is to provide highquality aluminium or aluminium alloy castings free from any fragments or bits of corroded parts of the casting machine.

A further object of the invention is to provide various durable components of a casting machine for aluminium or its alloy.

These objects are realized in accordance with this invention by carrying out the casting of aluminium or its alloy on a casting machine using components of sulphurized ferrous material for contact with molten aluminium or aluminium alloy during the casting operation. We have now found that a sulphurized surface layer of a ferrous material is extremely resistant to the corrosive attack of molten aluminium.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 schematically shows an example of a casting machine for carrying out the casting according to the invention; and

FIG. 2 is a micrograph of a section of the sulphurized surface portion of ferrous material.

DESCRIPTION OF A PREFERRED EMBODIMENT For the purpose of the invention the term "components of a casting machine is used to mean the mold,

crucible, conveying members, such as, ladle, gutter, pipe, and their accessories and fittings. At least the portions of these components that are brought into contact with molten aluminium or'aluminium alloy, and particularly those of such portions which are most easily cor roded by the molten metal, are impregnated with sulphur to form sulphurized surface layers in accordance with this invention.

The casting machine to be used for the practice of the invention, for the example for low pressure diecasting of aluminium or aluminium alloy, comprises, as shown in FIG. 1, a crucible l for containing molten metal 7, an air duct 6 for introducing air into the crucible to exert a pressure upon the surface of molten aluminium in the crucible, a mold 3 consisting of an upper mold member 3a and a lower mold member 3b and located above the crucible, a pipe 2 through which the molten metal is to be led into the cavity of the mold, a pyrometric instrument such as a thermocouple 4 for measuring the melt temperature, and a protecting tube 5 covering the thermocouple. Numeral 8 indicates a hydraulic cylinder for lifting the upper mold member 3a. The low pressure diecasting of aluminium or aluminium alloy according to the invention is performed in the following manner. First, ferrous components having sulphurized surface layers on at least those portions which will be exposed to the molten metal have to be provided. The components may have the sulphurized surface layers only on the portions which are usually corroded most easily. According to our experience, the parts most corrosive to the attacks by the melt are the outer surface of the pipe 2 and that of the protecting tube 5.

When aluminium or aluminium alloy is cast on a casting machine using the components guarded in the manner above described, the casting cycle can be repeated without the possibility of the parts being corroded or the part surface layers peeled off into the molten metal.

To attain the purpose of our invention it is important that the anticorrosion layers resistant to the molten aluminium or aluminium alloy should be formed integrally with the metal surfaces. Layers formed by mere covering or coating of the metal surfaces, as by plating, often come off during casting operation, and with such layers the objects of the present invention cannot be realized.

There is no special limitation to the means by which the surfaces of the components of a casting machine are impregnated with sulphur. One of the most effective means for sulphurizing a ferrous surface is to pretreat the surface by impregnation with iodine and then sulphurize the iodized layer. In this way, sulphur permeates very easily and deep into the metal and forms a molten aluminium-resistant surface layer of uniform thickness. For the sulphurization of ferrous components for casting used in the invention, the following method may be employed.

For the pretreatment above mentioned the process invented by one of the inventors of the present invention (as disclosed in U.S. Pat. No. 3,623,919) is adopted. The process consists in either immersing the portion of each ferrous part to be protected into an iodine solution or holding the same portion in an atmosphere of gasified iodine. The solution may be prepared by adding'iodine or an iodide, e.g., ammonium iodide (NHJ), sodium iodide (NaI), or aluminium iodide (Alla) to an organic solvent, e.g., ethyl alcohol (C- l l Ol-l), benzene (C 11 carbon disulfide (CS or acetone (CH COCl-l Alternatively, a water-soluble iodide, e.g., NHJ, Nal, or potassium iodide (Kl) may be dissolved in water. In either case, it is desirable that the solution is saturated with iodine or the iodide. By this pretreatment, an iodized surface layer is formed.

The iodized layers thus formed on the surfaces of casting machine parts are then immersed in molten sulphur or kept in contact with sulphur gas. The addition of not more than one percent by weight of iodine to the molten sulphur has the effect of increasing the thickness of the resulting sulphurized layer. By this treatment, the part surfaces are readily impregnated with sulphur. The outermost surface layer formed in this way is a Fe-S compound, and an iodized layer is interposed between this Fe-S compound layer and the substrate. A minor proportion of iodine present in the Fe-S compound layer does not impair the resistance of the surface layer to molten aluminium. Heat treatment of the surface layer thus obtained is contributory to further improvement of the layer resistance to molten aluminium.

A number of experiments thus far conducted have proven that the sulphurized layer formed in the foregoor 450C under a reduced pressure of l mm Hg for 3 hours.

The test pieces treated by (a) and (b) above were then immersed in molten metal baths of pure aluminium and an aluminium alloy (Al-l2wt.7rSi), and the period of time required for partial corrosion of the test piece surfaces were determined. For a comparison purpose, untreated test pieces and those coated wth alumina by plasma spraying to have a 0.3 mmthick alumina layer were also immersed in molten aluminium to determine the periods required for corrosion. The results are given in the table below. Test pieces of No. l to 16 in the table were pretreated with iodine according to the method described above (a) and dried, and then sulphurized and heat-treated under the conditions shown in the table. Test pieces No. 17 to 19 were untreated test pieces and No. 20, and 21 were alumina coated test pieces. Of the test pieces, No. l was a cast iron piece with its black skin directly sulphurized. All the rest (No. l to 16 except No. were mechanically ground and washed with trichloroethylene and then formed with a sulphurized layer on the surface. ln the column ofcorrosion resistance time": the value over 427, for example, means that the particular test piece exhibited no corrosion when it had been immersed in the molten aluminium for a period of 427 hours.

Test material Stainless steel (Fe-18% Cr8% Ni) Structural carbon steel (Fe-0.15% C0.2% Si-O.5% Mn) Structural carbon steel (Fe-0.45% CO.2% Si0.8% Mn) Cast iron (common cast iron) Stainless steel Structural carbon steel (Fe-0.15% C-0.2% Sl0.5% Mn) Sjtructural carbon steel (Fe-0.45% C0.2% Si0.8% Mn) ast iron Structural carbon steel (Fe-0.15% C0.2% Si0.5% Mn) Cast iron ing manner is extremely resistant to molten aluminium. The experiments performed are summarized below:

b. The test pieces obtained by the treatment under the same conditions as used above were baked at ZOOfC Heat treating Molten temp. alumin- C ium Corrosion resistance time (hi2) Test piece N o.

Sulphurizing time (hr.)

Alalloy 200 do 450 .do

Alumina coating (Plasma spraying) The results of the experiments above show that sulphurized layers have higher resistance to molten aluminium and aluminium alloys than ferrous materials and alumina coating, and also show that sulphurized layers which are heat treated have especially high resistance to molten aluminium and aluminium alloys.

EXAMPLE 1 About 0.1 mm-thick sulphurized layers were formed on the surface of the pipe for introducing molten aluminium alloy into the mold and also on the surface of the protecting tube of the thermocouple. The pipe and protecting tube were incorporated in a casting machine for low pressure casting as shown in FIG. 1.

Said pipe and protecting tube were both made of common cast iron having a black skin. Said pipe was 830 mm in length, 1 mm in outside diameter, and 80 mm in inside diameter. The protecting tube was 800 mm in length, 30 mm in outside diameter, and 16 mm in inside diameter.

Said pipe and protecting tube of common cast iron were immersed in a saturated solution of ethyl alcohol saturated with iodine for 6 hours, taken out and dried. Both were then immersed in molten sulphur containing 0.15 percent by weight of iodine for 24 hours. Next, they were taken out of the bath, dried, and heat treated at 200C under a reduced pressure of 10"mmHg for 3 hours. A sulphurized layer was formed on each surface.

An aluminium alloy (Al-9.5wt.%Si3.0wt.- %Cu-1.0wt.%Mg) in the molten state (at 720C) was cast on the machine having said pipe and protecting tube. One cycle of casting took 8 minutes, and the operation was continued for 6 hours a day. The protecting tube was in the state of immersion in the molten metal for 24 hours a day. After 10 days of repeated casting operations, not the slightest corrosion was observed. The protecting tube exhibited partial corrosion on the th day and the pipe on the 14th day.

Under the same conditions as above described, casting was carried out on a machine using a pipe and a protecting tube, both of common cast iron having black skin not sulphurized as above. The pipe and protecting tube showed signs of partial corrosion on the 4th to 5th day.

EXAMPLE 2 A ladle for diecasting was employed which was made of common cast iron having a wall thickness of 6 mm and dimensions of 360 mm X 300 mm X 260 mm. In the same manner as described in Example 1, a sulphurized layer about 0.1 mm-thick was formed on the ladle surface. Using this ladle, an aluminium alloy (Al-8.5wt.- %Si3.5wt.%Cu) was die cast at 650C. The ladle could carry about 8.5 kg of molten aluminium alloy at a time. The molten metal was thereby carried to the gate of a die casting machine and was poured from the ladle into the plunger'chamber. The ladle was operated for 12 hours and the afore-described procedure was repeated 480 times a day. On the 90th day the ladle showed a sign of partial corrosion.

On the other hand, ladies of common cast iron having a black skin but no sulphurized surface layer were used for the casting under conditions identical with those adopted for the above example. Partial corrosion was observed on the 7th to 10th day.

What is claimed is:

l. A method of casting aluminium or an aluminium alloy on a casting machine having components of a ferrous material characterized in that the casting is carried out using components which have an iodized layer and then a sulphurized surface layer on at least those portions which come into contact with the molten metal.

2. A method according to claim 1 wherein the components of the casting machine having the iodized and sulphurized layer are the casting mold, smelting crucible, gutter, ladle, pipe, and protecting tube for the pyrometric instrument for the molten metal.

3. A method according to claim 2 wherein the components of the casting machine are made of stainless steel, structural carbon steel, or cast iron.

4. A method according to claim 1 wherein the surface layers are formed by bringing at least the'portions of the components of the casting machine that contact molten aluminium or aluminium alloy into contact with a solution comprising an organic solvent and iodine or an iodide, a solution comprizing water and a water soluble iodide, or gasified iodine, thereby impregnating said portions with iodine, and then bringing the iodized portions into contact with either molten sulphur or sulphur gas so as to sulphurize said portions.

5. A method according to claim 4 wherein the organic solvent is selected from the group consisting of ethyl alcohol, benzene, carbon disulfide, and acetone.

6. A- method according to claim 4 wherein the iodide to be dissolved in the organic solvent is selected from the group consisting of ammonium iodide, sodium iodide, and aluminium iodide.

7. A method according to claim 4 wherein the iodide to be dissolved in water is selected from the group consisting of ammonium iodide, sodium iodide, and potassium iodide.

8. A method according to claim 4 wherein the molten sulphur into which the iodized portions of the components of casting machine are to be immersed contains not more than one percent of iodine.

9. A method according to claim 1 wherein the sulphurized portions are subjected to an additional step of heat-treating.

10. A casting machine for casting aluminium or an aluminium alloy comprising components of a ferrous material at least those portions of which are to be brought into contact with the molten metal have iodized and then sulphurized surface layers.

11. A casting machine according to claim 10 wherein the components having the sulphurized layer are the casting mold, smelting crucible, gutter, ladle, pipe, and protecting tube for the pyrometric instrument for the molten metal. 

2. A method according to claim 1 wherein the components of the casting machine having the iodized and sulphurized layer are the casting mold, smelting crucible, gutter, ladle, pipe, and protecting tube for the pyrometric instrument for the molten metal.
 3. A method according to claim 2 wherein the components of the casting machine are made of stainless steel, structural carbon steel, or cast iron.
 4. A method according to claim 1 wherein the surface layers are formed by bringing at least the portions of the components of the casting machine that contact molten aluminium or aluminium alloy into contact with a solution comprising an organic solvent and iodine or an iodide, a solution comprizing water and a water soluble iodide, or gasified iodine, thereby impregnating said portions with iodine, and then bringing the iodized portions into contact with either molten sulphur or sulphur gas so as to sulphurize said portions.
 5. A method according to claim 4 wherein The organic solvent is selected from the group consisting of ethyl alcohol, benzene, carbon disulfide, and acetone.
 6. A method according to claim 4 wherein the iodide to be dissolved in the organic solvent is selected from the group consisting of ammonium iodide, sodium iodide, and aluminium iodide.
 7. A method according to claim 4 wherein the iodide to be dissolved in water is selected from the group consisting of ammonium iodide, sodium iodide, and potassium iodide.
 8. A method according to claim 4 wherein the molten sulphur into which the iodized portions of the components of casting machine are to be immersed contains not more than one percent of iodine.
 9. A method according to claim 1 wherein the sulphurized portions are subjected to an additional step of heat-treating.
 10. A casting machine for casting aluminium or an aluminium alloy comprising components of a ferrous material at least those portions of which are to be brought into contact with the molten metal have iodized and then sulphurized surface layers.
 11. A casting machine according to claim 10 wherein the components having the sulphurized layer are the casting mold, smelting crucible, gutter, ladle, pipe, and protecting tube for the pyrometric instrument for the molten metal. 